The invention relates to a vessel-turret assembly comprising a hull structure for accommodating a turret, wherein bearing means are provided between said hull structure and said turret comprising axial bearing means supporting the weight of said turret, said axial bearing means comprising a raceway on either the hull structure or on the turret as well as rolling members running on said raceway, the turret being provided with a collar, said rolling members comprising at least three circumferentially spaced sets of bogies each comprising at least two rolling wheels rotating on shafts mounted in a bogie body comprising a supporting base structure which is connected to an attachment surface on either the hull structure or on the collar of the turret, wherein the other one of the hull structure or collar is provided with said raceway and wherein a displacement element is placed between said bogie body and the attachment surface.
Such a structure is generally known in the art. For offshore applications there is an increased tendency to the use of floating structures, such as vessels, to replace platforms for exploitation of oil wells. One reason is the lower costs involved whilst the other one is the possibility to provide a storage in the vessel proper.
To permit weathervaning of the vessel during connection with a sub sea oil well, a turret member is provided either inside the hull structure or at its bow or stern end. The vessel can rotate with regard to the turret and via the lower side of the turret, risers from one or more sub sea oil wells are received. On top of the turret a swivel is provided realizing a rotatable fluid connection between the turret and the vessel.
To optimize the use of sub sea oil wells and to optimize efficiency of the vessel the number of conduits extending through the turret as well as their diameter is increasing. In addition, for optimizing the production of the oil field, fluids, such as gas or steam, are injected.
To increase efficiency more conduits are connected to one turret and/or the diameter of each conduit is increased.
Consequently, there is an increasing need to larger diameter turrets. However, one of the limiting structural features is the axial bearing supporting a turret in the hull structure.
Up to now roller bearings have been used. The maximum diameter for such roller bearings is about 14 meters at the time being and the costs involved for such a bearing are high.
It must be borne in mind that at increasing the diameter of such bearing the fitting tolerances of several of the bearing parts and associated machine accuracy has to remain very high.
Because these bearings are custom made it can take up to a year after order before such a bearing is delivered. This means that after failure a turret structure comprising such bearings will be out of service during a considerable time.
It has been proposed to replace such a bearing by plain bearings. For example sliding pads could be used. Although it is less likely that such bearings will be damaged and such bearings are much cheaper, the weathervaning properties of the vessel dramatically decrease. This means that during a first part of rotation of the vessel because of changing wind and waves the turret will move along with the vessel. The anchor chains connected to the turret are subjected to considerable rotational forces which means that such chains, which are relatively heavy are subject to interlink wear and consequently to unforeseen failure.
From WO 93/07049 a turret-assembly is known in which the turret in axial direction is supported on a plurality of bearing arms which carry axial and radial bearing elements. The axial bearing elements are formed by bogie wheels which are supported on a double raceway on the vessel. The bogie wheels are mounted in a bracket which is connected to the radial arms by means of a rubber filler which eliminates sliding movements in the bearings of the wheels in the bogies and will help even out the load on the wheels.
To each arm a support wheel is connected having its rotation axis aligned in the axial direction. The circumferential surface of the guide wheel runs along the hull structure. Even though movement of the turret in the radial direction can be prevented to a certain extent, it is still possible that the bogie wheels may move relative to the raceways. This can lead to an increased wear and reduced service life. It is therefore an object of the present invention to allow for relative movements of the turret and the hull structure, while maintaining an accurately defined position of the load-bearing surfaces of the bogie wheels on the raceways.
The known construction has as a further disadvantage that in case of failure of a bogie, it will be difficult to replace. This will mean that the known turrets will be out of service for a relatively long time when failure of a bogie occurs.